The hydrogenation of carbon dioxide by hydrogen has been known and industrialized as a method for the production of a hydrocarbon using a precious metal (e.g., Ru, Rh) type catalyst or a Ni type catalyst, as shown in the following reaction formula. According to this reaction, methane can be readily produced with high selectivity, and it hardly produces CO. EQU CO.sub.2 +4H.sub.2 .revreaction.CH.sub.4 +2H.sub.2 O
On the other hand, carbon monoxide alone or mixed with hydrogen in an equimolar amount (called oxo gas) is useful as a raw material for methanol synthesis, acrylic acid synthesis, formic acid synthesis, fatty acid synthesis, acetic acid synthesis, oxo synthesis (hydroformylation), and carbonyl synthesis, etc.
In general, carbon monoxide is produced by the steam reforming process of a light hydrocarbon.
In the steam reforming process, a light hydrocarbon (e.g., methane), water and carbon dioxide are reacted in the presence of a catalyst to change the reactants to a gas containing H.sub.2 /CO.sub.2 /CO, then CO.sub.2 in the gas is absorbed by an amine solution and the like, whereby a mixed gas of H.sub.2 with CO is obtained, or which may be further deeply cooled to separate CO.
Recently, in the production of carbon monoxide, research has become active to solidify and utilize CO.sub.2 as resources in view of global environmental protection. For this purpose, research has been conducted to develop a catalyst which is capable of producing CO with high selectivity by the reduction of carbon dioxide as a raw material using hydrogen, as shown in the following reaction formula. EQU CO.sub.2 +H.sub.2 .revreaction.CO+H.sub.2 O
This reaction is to selectively produce CO without forming a hydrocarbon.
A catalyst used in this reaction is required to have a degree of conversion to the equilibrium degree of conversion as its activity and high selectivity, so such a catalyst is severely difficult to design. Accordingly, if any sulfur compound such as H.sub.2 S is in a raw material gas, the catalyst is poisoned instantly with a sulfur compound.
As an improved catalyst for this purpose, JP-A-4-363142 discloses a tungsten sulfide catalyst and a molybdenum sulfide catalyst. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
These catalysts are prepared by previously treating ammonium tetrathiotungstate (NH.sub.4).sub.2 WS.sub.4 ! or ammonium tetrathiomolybdate (NH.sub.4).sub.2 MoS.sub.4 ! under H.sub.2 stream at 300 to 400.degree. C. to prepare WS.sub.2 or MoS.sub.2.
Also, there are catalysts such as MoS.sub.2 /TiO.sub.2, MoS.sub.2 /Al.sub.2 O.sub.3 which are prepared by dipping a carrier such as TiO.sub.2, Al.sub.2 O.sub.3, SiO.sub.2 in the above-mentioned aqueous ammonium sulfide solution to which an aqueous ammonia is added for support, followed by drying and pretreatment.
These catalysts are not poisoned with a sulfur compound, since a metal sulfide is used as a catalyst active component.
Moreover, it is known that these catalysts are capable of producing CO in the reduction reaction of carbon dioxide using a mixed gas of carbon dioxide and hydrogen with high selectivity without forming a hydrocarbon.
Furthermore, since these catalysts do not suffer the poisoning function from a sulfur compound as discussed above, they are also advantageous in that no removal of H.sub.2 S is required.
However, in cases where a large amount of CO is contained in a raw material gas, it is generally known, as shown in the following reaction formulas, that they remarkably form a hydrocarbon and/or deposit a carbon, further they bring about the deactivation of these catalysts due to CO. EQU CO+(m/2n+1)H.sub.2 V0.fwdarw.1/n.times.CnHm+H.sub.2 O (formation of a hydrocarbon) EQU 2CO.fwdarw.CO.sub.2 +C.arrow-down dbl. (deposition of a carbon)
In addition, in cases where a slight amount of sulfur compound is contained or no sulfur compound is contained in a raw material gas, a metal sulfide as a catalyst active component is reduced with hydrogen to form H.sub.2 S in the reaction of carbon dioxide with hydrogen, then the H.sub.2 S is transferred to a reaction product in the CO.sub.2 --H.sub.2 system, and, as a result, the catalyst is deactivated.
Furthermore, regardless of the presence or absence of a sulfur compound, such as H.sub.2 S, in a raw material gas, any post-treatment for removing H.sub.2 S is required due to the transferred H.sub.2 S in the reaction product.
On the other hand, a reaction gas is condensed and circulated after separation of a great amount of unreacted CO.sub.2 in the process of forming an oxo gas (CO/H.sub.2 =1) by the steam reforming process and in the process of separating CO conducted by deeply cooling the resulting oxo gas. Accordingly, when a sulfur compound such as H.sub.2 S is slipped into a formed gas in the course of the reduction of a catalyst, the sulfur compound is condensed at the same time in the course of CO.sub.2 separation and CO.sub.2 condensation steps and the condensed sulfur compound is introduced to a reforming reactor, then a reforming catalyst is unfavorably poisoned, which is observed in using a sulfide catalyst. In this process, it is important to decrease an amount of unreacted CO.sub.2 for cost saving. For this purpose, it is essentially required to conduct the reverse shift reaction of a reforming gas containing CO, CO.sub.2, H.sub.2 to decrease the amount of CO.sub.2 in the gas. Generally, in cases where CO is present, it brings about remarkable catalyst poisoning, carbon deposition, and hydrocarbon formation, etc.